This invention relates to the field of drop marking systems of the type in which a liquid ink is forced under pressure through a nozzle which converts the liquid into droplets which can then be controlled by various means while projected toward a substrate for marking purposes. Examples of such systems include the familiar ink jet marking systems used for high speed label printing, product identification and the like, although there are other drop marking systems known in the art. One particular type of system which advantageously employs the present invention is the continuous stream, ink jet printer. Such a system typically includes an ink reservoir and a remotely located nozzle connected to the reservoir by a conduit. Ink is forced under pressure from the reservoir to the nozzle which emits a continuous stream of ink drops. The ink, which is electrically conductive, is provided with a charge as the drops leave the nozzle. The drops then pass through a deflection field which causes selected drops to be deflected so that some of the drops are deposited onto a substrate while the remaining drops are returned to the reservoir by a suitable ink return means.
In order to produce high quality marking, it is important that the ink is maintained at its formulated concentration of nonevaporative solids. Ink drop formation, drop electrical charge, drop velocity, spot placement accuracy, spot placement precision, spot adhesion, spot drying time, and spot optical properties are printing parameters that have some dependence on ink properties. These ink properties include composition, electrical conductivity, density, acoustic velocity, surface tension, and viscosity. These ink properties have a dependence on solids concentration. So, if ink solids concentration deviates from specifications, the print quality may also deviate from acceptable standards.
Print quality is highly dependent on drop velocity. In turn, drop velocity is dependent on ink viscosity. Ink viscosity is highly dependent on ink solids concentration. Thus, drop velocity and print quality are strongly dependent on ink solids concentration.
The condition of constant ink drop velocity through the deflection field requires that the flow rate of liquid through the nozzle be substantially constant. Prior ink marking systems have attempted to accommodate this requirement by various means.
One such system employs a specific gravity detector which signals when it is necessary to add solvent to the ink supply. This system is unsuitable for use in systems where the printer must accommodate many different types of inks, each with its own specific gravity parameters.
Another commercial system which tries to deal with the problem of changing drop velocity was manufactured by the IBM Corporation. In this device the ink pressure is responsive to signals from a deflection detector. The deflection detector is located in the electric field through which the drops pass. The detector signals the pump to increase or decrease pressure, as necessary, to maintain drop velocity at an appropriate value. The system provides feedback control of drop velocity. The technique, however, is not entirely satisfactory because of the complexity and cost of the components and the need for a fragile deflection detector at the remote print head location.
Another invention, disclosed in U.S. Pat. No. 4,555,712, monitors the ink flow rate, monitors the velocity of the drops of ink in the charge field and, by use of an electronic controller, adjusts the ink parameters to maintain a desired flow rate which insures a substantially constant drop velocity.
It is an object of the present invention to incorporate direct feedback control into an ink solids concentration control system which is simpler, reliable and low in cost.
Another object of the invention is to provide a velocity control system for an ink jet printer which maintains substantially constant velocity of ink entering a deflection field thereby insuring accurate location of spots on the substrate to be marked.
A further object of the invention is to provide an electronic control system employing acoustic transducers to measure the velocity of sound in ink to permit accurate control of the addition of solvent to the ink.
Another object of the invention is to provide a flow control means for an ink system which is located entirely separate from the print head nozzle and yet maintains a substantially constant flow rate through the nozzle.
Other objects and advantages of the invention will be apparent from the remaining portion of the description.